A forming roll of a double cylinder structure is known as one of forming rolls for forming plastic sheets, films and the like. As disclosed in Patent Literature 1, the forming roll of a double cylinder structure has a highly rigid metallic internal cylinder arranged inside and coaxially with a flexible thin metallic external cylinder with a space for circulating a coolant provided between the internal and the external cylinders.
Patent Literature 1 states that the very practical thickness of a thin cylinder body used as the thin metallic external cylinder of the forming roll of this type is within a range from a lower limit of 2 mm due to machining constraints, to an upper limit of 5 mm.
However, in recent years, there have been more strict demands that the thin cylindrical body be thinner and achieve higher processing accuracy in order to form more advanced plastic sheets and films. Thus, holding the thin cylindrical body in machining processing is required to fully respond to the demands.
As holding jigs for holding a thin cylindrical body of this type (hereinafter referred to as a “thin cylindrical work”) in the machining processing, holding jigs described in Patent Literatures 2 and 3 have been known. In each of the holding jigs described in Patent Literatures 2 and 3, an elastic member made of a rubber or the like is provided between an outer periphery of a holding member inserted to an inner periphery of a work and the inner periphery of the work, and a force to hold the work is generated by compressing the elastic member in an axial direction and thereby expanding the elastic member in a radial direction.
In this case, the elastic member is made of a soft material such as rubber. When the material is used over the full length in the axial direction, a compressive force is not distributed to the entire elastic member even if such a force is applied from both ends of the elastic member. Therefore, the elastic members and spacers are alternately arranged.